Formation of Micrometer‐Sized Textured Hexagonal Silicon Crystals via Nanoindentation
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2025-03-02 |
| Journal | Small Structures |
| Authors | M. Bikerouin, Anna Marzegalli, Davide Spirito, Gerald J.K. Schaffar, Corrado Bongiorno |
| Institutions | Institute for Microelectronics and Microsystems, Roma Tre University |
| Citations | 2 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This study successfully demonstrates a scalable, controlled method for synthesizing high-quality, micrometer-sized hexagonal diamond silicon (hd-Si) crystals, a metastable phase crucial for future optoelectronics.
- Core Achievement: Successful, reproducible formation of micrometer-sized hd-Si (Si-IV) crystals in silicon substrates using a two-step process: nanoindentation followed by low-temperature annealing.
- Phase Transformation Pathway: Mechanical pressure transforms diamond cubic silicon (dc-Si) into high-density metastable phases (bc8/r8 mixture), which subsequently convert completely into hd-Si upon annealing at 250 °C.
- Structural Quality: The resulting hd-Si exhibits high crystal quality, evidenced by narrow Raman peak widths and large, uniform transformed domains (Group-I pits) free from cracks and major defects.
- Unique Texture: The hd-Si is not monocrystalline but a textured assembly of nanometer-sized grains (5-30 nm) organized into large, micrometer-scale domains sharing a common crystallographic orientation.
- Process Control: Optimization using a 20 µm spherical indenter and slow unloading rates minimized plastic relaxation and cracking, leading to a high statistical yield (70%) of superior transformed regions.
- Optical Properties: EELS analysis confirms phase-dependent optical properties, showing a plasmon peak redshift for hd-Si (16.7 eV) compared to dc-Si (17.0 eV), suggesting potential for tailored optical applications.
- Engineering Significance: The textured hd-Si structure offers a promising template for the epitaxial growth of direct-bandgap hexagonal SiGe alloys, enabling integration into silicon-based photonic circuits.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Indenter Tip Radius | 20 | µm | Spherical diamond tip used for optimized Group-I pits. |
| Maximum Load (Pmax) | 665 | mN | Applied load for 20 µm tip experiments. |
| Loading Strain Rate | 10-3 | s-1 | Constant strain rate used during indentation. |
| Unloading Rate | 1 | mN s-1 | Slow unloading rate used to favor crystalline transformation over amorphization. |
| Phase Transformation Onset Pressure | 10-11 | GPa | Pressure range where dc-Si transitions to metallic Si-II begins. |
| Annealing Temperature | 250 | °C | Performed in a quartz tube furnace under nitrogen atmosphere. |
| Annealing Time | 2 | hours | Duration of the thermal treatment. |
| hd-Si E2g Raman Mode (Exp.) | 487.2 ± 7.5 | cm-1 | Experimental frequency of the hexagonal diamond phase. |
| dc-Si T2g Raman Mode (Exp.) | 527.0 ± 3.2 | cm-1 | Experimental frequency of the diamond cubic phase (substrate). |
| hd-Si Plasmon Peak (Exp.) | 16.7 | eV | Low-loss EELS peak, indicating a redshift from dc-Si (17.0 eV). |
| dc-Si Plasmon Peak (Exp.) | 17.0 | eV | Low-loss EELS peak for the diamond cubic phase. |
| hd-Si Grain Size | 5-30 | nm | Size range of nanometer-sized grains forming the textured domains. |
| Group-I Pit Yield | 70 | % | Statistical occurrence of pits exhibiting uniform, crack-free transformed regions (using 20 µm tip). |
Key Methodologies
Section titled “Key Methodologies”The synthesis and characterization relied on a precise two-step process (nanoindentation and annealing) coupled with advanced structural and spectroscopic analysis.
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Nanoindentation (Phase Induction):
- Substrate: Monocrystalline silicon (001).
- Indenter: Spherical diamond tips (20 µm radius) were primarily used to distribute stress over a larger area, reducing peak stresses and minimizing cracking risk compared to sharp indenters (e.g., Berkovich).
- Loading Parameters: Load applied up to 665 mN at a constant strain rate of 10-3 s-1. This ensured the transformation pressure (10-11 GPa) was reached without inducing “pop-in” events (which indicate cracking/plastic relaxation).
- Unloading Parameters: Slow unloading rate (1 mN s-1) was used to favor the formation of stable intermediate crystalline phases (bc8/r8 mixture) rather than amorphous silicon (a-Si).
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Annealing (Phase Stabilization):
- Environment: Quartz tube furnace under nitrogen (N2) atmosphere.
- Conditions: Samples were annealed at 250 °C for 2 hours. This thermal treatment drove the complete transformation of the metastable bc8/r8 mixture into the stable hexagonal diamond (hd-Si) phase.
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Structural and Spectroscopic Characterization:
- Raman Spectroscopy (Polarized and Nonpolarized): Used to identify the crystal phases (bc8, r8, hd, dc) and determine their crystallographic orientation. Polarized Raman confirmed the coherent orientation (texture) of the metastable phases.
- High-Resolution Transmission Electron Microscopy (HR-TEM) and Selective Area Electron Diffraction (SAED): Used to confirm the presence of hd-Si crystals, analyze the microstructural features (nanometer-sized grains), and verify the textured domain structure within the micrometer-sized pits.
- Electron Energy-Loss Spectroscopy (EELS): Used to confirm the phase transformation by measuring the bulk plasmon excitation energy, which showed a characteristic redshift for hd-Si (16.7 eV).
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Computational Modeling:
- First-Principles Calculations (DFT): Used to calculate theoretical Raman spectra, energy differences, and equilibrium volumes for dc, r8, bc8, and hd phases, validating experimental observations.
- Molecular Dynamics (MD) Simulations: Employed machine learning (ML)-based interatomic potentials to model the atomistic mechanism of the bc8/r8 mixture transforming into hd-Si during the thermalization step.
Commercial Applications
Section titled “Commercial Applications”The successful, controlled synthesis of textured hexagonal silicon (hd-Si) opens pathways for integration into advanced semiconductor and optoelectronic technologies.
| Application Area | Technical Advantage of hd-Si | Related Industries/Products |
|---|---|---|
| On-Chip Photonics | Predicted direct bandgap properties enhance light emission and detection capabilities, overcoming the limitations of indirect-bandgap dc-Si. | Silicon photonics, integrated optical circuits, optical interconnects. |
| Epitaxial Growth Template | hd-Si’s thermal stability (greater than 700 °C) and hexagonal structure make it an ideal template for growing direct-bandgap hexagonal SiGe alloys. | High-efficiency LEDs, lasers, and photodetectors monolithically integrated on Si substrates. |
| High-Mobility Electronics | Certain Si polymorphs, including hd-Si, are predicted to exhibit high carrier mobility. | Advanced CMOS technology, high-speed transistors, next-generation electronic devices. |
| Energy Harvesting | Potential for low thermal conductivity and high thermoelectric efficiency compared to dc-Si. | Thermoelectric generators (TEGs), waste heat recovery systems, on-chip power sources. |
| Advanced Materials Engineering | Nanoindentation is validated as a precise, versatile tool for inducing pressure-driven phase transformations and stabilizing novel metastable materials. | Materials synthesis, strain engineering, development of novel silicon allotropes for specific electronic band structures. |
View Original Abstract
A comprehensive study on the formation of micrometer‐sized, textured hexagonal diamond silicon (hd‐Si) crystals via nanoindentation followed by annealing is presented. Utilizing advanced characterization techniques such as polarized Raman spectroscopy, high‐resolution transmission electron microscopy, and electron energy‐loss spectroscopy, the successful transformation of silicon into high‐quality hd‐Si is demonstrated. The experimental results are further supported by first‐principles calculations and molecular dynamics simulations. Notably, the hd‐Si phase consists of nanometer‐sized grains with slight misorientations, organized into large micrometer‐scale textured domains. These findings underscore the potential of nanoindentation as a precise and versatile tool for inducing pressure‐driven phase transformations, particularly for the stabilization of hexagonal silicon. The textured nature of hd‐Si also presents a unique opportunity to tailor its optical properties, opening new avenues for its application in semiconductor and optoelectronic devices.